Mass Effect series
Assassins Creed series (although they are all more or less the same in terms of gameplay, they are still fun)
Prototype
The two Batman games (Arkham Asylum and the other one) are awesome too.
Two Worlds 2 for a good hack 'n' slash / looter RPG with nonsense story and silly men with long blonde hair.
And if you have a console nothing beats Red Dead Redemption - gotta love that lasso!
Gears of War 1-2-3
Mass Effect 1-2
Beyond Good and Evil (classic)
Gothic 1-2-3 (especially 2 - classic)
Neverwinter Nights 2
Fahrenheit (indigo prophecy) (classic)
Warhammer 40K: Space Marine
Hitman 1-2-3-4 (especially 4) (classic)
Splinter Cell 1-2-3-4-5
inFamous 1-2
Saint's Row 3
Dead Space 1-2
Red Dead Redemption
GTA 4
Batman 1-2
Alan Wake
Alpha Protocol
LA Noire
Resident Evil 5 (though I prefer 3)
Assassin's Creed 1-2-2.2-2.3 (2 is the best)
Star Trek Online *evilgrin*
Just Cause 2 (uber fun)
Some of the games that I played and liked and that has 3rd person view. They are mostly action - rpg - sandbox.